1. Technical Field
The subject invention relates to a method and apparatus for making cam shafts wherein cam lobes are disposed about and secured to a hollow tubular shaft.
2. Description of the Related Art
Cam shafts used for internal combustion engines generally consist of several cam lobes which are rigidly attached to a common shaft. These individual lobes normally activate intake or exhaust valve cam followers and, in turn, control the valves either by direct contact therewith or by way of push rods. The basic cam shaft has remained virtually unchanged in construction since the first multi-cylinder internal combustion engine of one hundred years ago.
There are several variants of traditional methods for commercially manufacturing cam shafts. Most methods involve the rough casting (in iron) of the cam shaft assembly, including the shaft, cam lobes, and bearing journals within a sand mold. Generally, the rough castings are ground in automatic machines to correctly profile the cam lobe surfaces, which not only have to be smooth and of a given curvature, but each lobe must also be in the correct disposition relative to the other cam lobes on the shaft. After the grinding operation which is slow and expensive, the cam lobes must be hardened to improve the wear resistance of the cam surfaces. Commonly, the cams are induction heat treated to insure that the proper hardness has been obtained without inducing distortions. The cams are then finished, ground and parkerized to enhance lubricant retention and to reduce the break-in interval time period. Some manufacturers pre-harden the cast blank (ungrounded lobes) to a substantial depth (0.08 to 0.09 inches) to permit uninterrupted rough and finish grinding.
Recently, all steel cam shafts have been used in some production engines because of the improved performance associated therewith. Tubular shafts permit weight savings (10%-20%) and the better metallurgical properties of the alloy steel permit improved surface hardness to be achieved with out compromise of toughness. Cast steel or billet derived cam shafts are quite expensive and show up most frequently in premium higher output engines. In an effort to obtain the advantages offered by steel cam shafts at costs more in line with cast iron cams, various manufacturers have experimented with assembled cam shafts. Assembled cam shafts generally consist of a tubular steel shaft onto which a plurality of cam lobes are secured in their proper orientations. With such assemblies, the cam lobes may be manufactured in large numbers and ground to their end product finish and curvature without being hampered by the shaft.
Manufacturing techniques which expand the tubular shaft outward to engage and retain cam lobes disposed thereabout is an attractive method for making cam shafts because all of the lobes can be fastened to the shaft at essentially one time. Generally, a holding fixture indexes all of the pre-ground cam lobe surfaces with great precision, thus making the process very repeatable. A major difficulty with such tube expanding methods, however, is the force required to expand the tube shaft.
One method that has been used to deform the shaft outward to engage the cam lobe is the "ballizing" method as shown in U.S. Pat. No. 4,575,913 to Sugiuchi. The "ballizing" method includes forcing a lubricated mandrel through the interior of the hollow shaft thereby expanding the outer diameter of the shaft and forcing the outer shaft into an interference fit within the inner diameter of an aperture disposed through each cam lobe. Circumferencial serrations located around the apertures of each cam lobe act as a spline which greatly enhances torsional stiffness by locking the lobe to the shaft. This "ballizing" technique is adaptable to commercial cam shaft manufacturing, but the extremely high friction of the mandrel against the inner shaft wall causes metal fretting and extreme wear. The life of the mandrel is therefor severely limited thus requiring uneconomical tool replacement after every few parts.
Another shaft expanding method is shown in U.S. Pat. Nos. 4,738,012; 4,763,503, 4,693,138; and 5,085,099 all in the name of Hughes, which rely upon a similar cam lobe fastening topology, but accomplish the shaft expansion by hydraulic means. More specifically, the inner volume of the shaft is filled with a hydraulic fluid which acts as the working and pressure distributing fluid. A displacer piston rod is forced into the interior space of the shaft thereby increasing the internal pressure. This pressure increase will plasticly deform the steel shaft by locally yielding the wall and allowing the strained metal to mechanically lock the lobes in place.
Still another shaft expanding method is shown in U.S. Pat. No. 3,845,667 to Honrath and 3,869,938 to Schlotterbeck which involves the use of electromagnetic induction wherein an electronic discharge creates a shock wave to expand the shaft in localized areas. Unfortunately, such electromagnetic techniques require extremely expensive equipment. Moreover, when using shafts of substantial thickness, the expense involved in creating the necessary electronic field quickly becomes so great as to be impracticable.
English Patent No. 1,117,816 to Allsop briefly suggest that explosive forming techniques may be used to create pressure for expanding the shaft into engagement with cam lobes disposed axially therealong. However, the Allsop '816 patent discloses no specific explosive forming technique.
U.S. Pat. No. 3,869,938 to Schlotterbeck also discloses a specific explosive forming method wherein a beryllium-copper electrode is ignited within an external explosion chamber, thus releasing a shock wave which is transferred into the shaft through a water medium thereby expanding the shaft outward. Such exploding electrode techniques typically involve initiating the electrode to detonate thus creating a plasma shock wave.
Methods for expanding cam shafts by way of detonating explosives have many drawbacks. The process of detonation creates pressure shock wave wherein extremely high peak pressures are attained almost instantaneously. Consequently, the inner diameter in closes proximity to the explosive or pressure shock wave tends to crack or become extremely stressed due to the peak pressure of the shock wave. Moreover, due to the high pressure shock wave, localized areas of the shaft often become thinner as a result of material flow.
The present invention addresses the deficiencies of the current technology by retaining the convenience and cost saving features of the assembled cam lobe and shaft design, with a new means for expanding the shaft.